Frequency to voltage converters are required to perform linear, low-distortion and highly sensitive conversion of frequency to voltage. Conventionally, many kinds of frequency to voltage converters are available.
One kind of frequency to voltage converter is disclosed in U.S. Pat. No. 3,784,845, which includes a squaring amplifier responsive to an applied input frequency signal for producing constant amplitude square waves at the input frequency. A first capacitor is rapidly charged from a reference voltage and a voltage due to an accumulated charge on a second capacitor during the first half cycle of each square wave. The charge accumulated on the first capacitor is rapidly transferred to the second capacitor during the second half cycle of each square wave. The voltage due to the accumulated charge on the second capacitor is coupled to a buffer amplifier through a long time constant integrating filter whereby the amplifier provides an output D.C. voltage which is linearly proportional to the frequency of the input signal. However, the output DC voltage is proportional to the input frequency only when the discharging time constant of the second capacitor is large compared to an internal time constant.
Another type of frequency to voltage converter is a PLL-based frequency demodulator. Though PLL-based demodulators can ensure linear conversion of frequency to voltage once PLL is locked, the lock range of PLL is determined by the loop bandwidth. In order to attain good tracking characteristics by reducing transient error of input modulated wave and to reduce the jitter in the output due to internal VCO noise, the loop bandwidth should be more compared to modulation bandwidth. However, when C/N ratio of the input signal is inferior, the loop bandwidth has to be kept minimum in order to reduce the jitter at the output. Therefore, it is difficult to attain both good tracking characteristics and a low-distortion demodulation of input frequency signals using above mentioned PLL-based frequency demodulator.
Yet another type of frequency to voltage converter is disclosed in U.S. Pat. No. 4,479,091 titled as Phase locked loop FM demodulator with variable bandwidth loop filter. In said document, the loop bandwidth is varied by a control voltage generated from a signal proportional to modulation index and a signal proportional to noise. Hence linear demodulation is made possible even in the presence of external noise. However, the range through which the bandwidth is varied is limited as the loop bandwidth is controlled by the channel resistance of JFET which saturates beyond pinch off. Moreover multiple feedback loops are required.
Yet another kind of frequency to voltage converter is available which facilitate conversion of frequency to current and then current to voltage using a resistive load. In the design of linear integrated circuits of such frequency to voltage converters, it is desirable to have an electrical resistance load element which is linear, that is, where the current through the load is linearly proportional to the voltage across the load over a reasonably wide operating range of voltage. However, using such a load consumes an inordinately large amount of semi-conductor silicon wafer area. On the other hand, use of source to drain resistance of an insulated gate metal oxide semiconductor field effect transistor (IGFET or MOSFET) as a load enables more compact implementation of such a load but only at the expense of non-linearity over desired operating parameters.
Hence, there is a need for a frequency to voltage converter designed using CMOS technology which facilitates linear, low-distortion conversion of frequency to voltage and is also capable of detecting input signals of low frequency deviations.